Truck and bus suspensions are commonly designed using a pair of leaf springs between both the front and rear axles of the vehicle (the unsprung portion) and the body of the vehicle (the sprung portion). The leaf springs are normally a plurality of arcuately shaped steel or composite leafs that are stacked together to form the leaf spring. The axle of the vehicle is normally secured to the approximate center of the arcuate leafs with the end of the leafs extending upwards. The upward end of the leafs are normally formed into a tubular section or eye which is adapted for receiving a spring pivot bushing. The spring pivot bushing usually consists of an outer metal housing which is pressed into the eye of the spring, a layer of elastomer positioned within the outer metal housing and an inner metal housing which extends through the center of the layer of elastomer. A bolt extends through the inner metal and secures the end of the leaf spring to the frame or sprung portion of the vehicle by mating with an appropriate bracket. As the vehicle travels, relative movement between the sprung and unsprung portions of the vehicle is accommodated by flexing of the leaf springs. The flexing of the leaf springs causes the ends of the leaf springs to pivot on both of the tubular sections or eyes which secure the leaf spring to the sprung portion of the vehicle.
The spring pivot bushings are used to facilitate this pivotal motion and to isolate the vehicle from shock. The layer of elastomer located between the inner and outer metal housings effectively isolates the sprung portion of the vehicle from the unsprung portion of the vehicle. In certain high load applications, the ends of the outer metal are curved over towards the inner metal in order to further encapsulate the layer of elastomer. The curving of the ends and thus the further encapsulating of the layer of elastomer improves the radial spring rate, it improves the axial spring rate, it improves the axial retention and it improves the durability of the bushing.
While these elastomer isolated pivot bushings have performed satisfactorily in the field, they are not without their problems. The various problems associated with these prior art pivot bushings include variations in the diameters of the spring ends and distortion of the cross section in the area where the pivot bushing is pressed into the spring ends. These manufacturing variations in the configuration of the spring end, often allow the bushing to slip out of the spring when the spring undergoes an axial load.
Also, in the higher load applications, it is not uncommon for the outer metal to split due to high loads. This splitting of the outer metal can be avoided by heat treating of the outer metal. However, the outer metal in the higher load applications must remain soft in order for it to be crimped over. Thus, with curled end bushings, the heat treatment of the entire bushing is not a possibility. Another option for improving the strength of the outer metal is to manufacture the outer metal from drawn over mandrel (DOM) tubing which is superior in strength. While this DOM tubing will increase the strength of the tubing, it also significantly increases the manufacturing costs associated with the pivot bushing.
Thus, the continued development of pivot bushings has been directed to the improvement of performance, strength and durability while minimizing the manufacturing costs associated with the pivot bushing.